What will speed up renders more effectively?- more cores or higher clock speed?
From what I read about in the past, more cores give you "better" rendering because of the parallelism. Regarding GPU rendering, the thing you also have to consider is whether you want this to be scalable and if your scene has a large memory footprint. Running out of GPU VRAM will essentially leave you without a GPU. If you're building a renderfarm, cores are the way to go. Plus, you can set the render to leave one or two cores free for other applications to use, so you'd be left with 4 or 5 cores on the 6, versus 2 cores on the 4. I personally use a 6-core Phenom II OCed to 3.72 GHz, which does everything I need it to and I can run background applications without issue. I can set the tile size up and get a reasonable looking preview pretty quickly without having to wait for two tiles at a time. Then, when I need to do a final render, I just set it back up to 6 cores and let it sit overnight or while I'm out. 

tl;dr - I would say go with the 6.